1. Field of the Invention
The present invention relates to a photovoltaic element such as a solar cell and a process for the production thereof. More particularly, the present invention relates to a highly reliable photovoltaic element having an improved electrode structure which excels in durability and a process for the production thereof.
2. Related Background Art
In recent years, mainly aiming at reducing pollution gas in the air which causes a global-warming phenomenon, the introduction of a sunlight power generation system has been increasing. Under this circumstance, it is necessary to provide highly reliable photovoltaic elements for use in such sunlight power generation system at a reasonable cost.
As an example of such photovoltaic element which can be provided at a reasonable cost, Japanese Laid-open Patent Publication No. Hei.9(1997)-36395 (hereinafter referred to as document 1) discloses a photovoltaic element having such a configuration as shown in FIGS. 2(a) and 2(b). FIG. 2(a) is a schematic view illustrating a principal portion of an electrode structure of the photovoltaic element on the light incident side. FIG. 2(b) is a schematic cross-sectional view, taken along the line B-Bxe2x80x2 in FIG. 2(a).
In FIGS. 2(a) and 2(b), reference numeral 501 indicates a photovoltaic element, reference numeral 502 a bus bar electrode comprising a belt-like shaped metal foil, reference numeral 504 a collecting electrode comprising a plurality of metal wires, and reference numeral 505 a joining portion to join the bus bar electrode 502 and the collecting electrode 504. The collecting electrode herein is generally called a finger electrode to the bus bar electrode.
The electric current generated by the photovoltaic element 501 is collected by the collecting electrode 504, followed by being flown to the bus bar electrode 502 while passing through the joining portion 505 where the electric current is collected by the bus bar electrode 502, followed by being guided to the outside.
In the case of a photovoltaic element having such a configuration as shown in FIGS. 2(a) and 2(b) having an electrode structure comprising such bus bar electrode which comprises a belt-like shaped metal foil and such collecting electrode which comprises a plurality of metal wires, the electrode structure is capable of allowing a large quantity of an electric current to flow therein because the electrode structure is relatively small in terms of the electric resistance. Thus, it is considered that when the photovoltaic element is designed to have a large area (a large size in other words), it is possible to increase the quantity of an electric current generated by the photovoltaic element and such photovoltaic element having a large area can be more efficiently produced in comparison with the case of producing a photovoltaic element having a relatively small area, where the photovoltaic element having a large area can be produced at a reduced production cost.
Therefore, it is expected that the technique disclosed in document 1 will be possible to realize a photovoltaic element having a large area and such electrode structure as shown in FIGS. 2(a) and 2(b) which has such advantages as above described at a reasonable production cost.
However, for the photovoltaic element having such electrode structure as shown in FIGS. 2(a) and 2(b) which is disclosed in document 1, the present inventors have found that it has a disadvantage such that as the quantity of an electric current generated by the photovoltaic element is increased, the quantity of the electric current which flows in the collecting electrode is also increased to deteriorate the performance of the photovoltaic element. And in order to eliminate this disadvantage, the present inventors have found that it is important to decrease the electric resistance particularly of the joining portion (505) of the electrode structure so as to diminish the power loss in the joining portion in order to prevent the performance of the photovoltaic element from being deteriorated.
In order to achieve this aim, it is considered to be effective that the electric resistance of the joining portion is decreased by making the joining portion to comprise a combination of a carbon paste and an electrically conductive paste comprising an electrically conductive filler and a binder and which has excellent electric conductivity.
However, this method has found to be difficult to attain a reduction in the electric resistance of the joining portion and an improvement in the strength of the joining portion at the same time. For the reason for this, it is considered such that in order to obtain an electrically conductive paste, it is necessary to increase the ratio of the electrically conductive filler to the binder, but then the ratio of the electrically conductive filler is increased in this way, the adhesive strength of the electrically conductive paste is decreased.
The present invention is aimed at solving the foregoing problems in the prior art and achieving a highly reliable photovoltaic element having an improved electrode structure in which the power loss due to the electric resistance upon collecting an electric current from the photovoltaic element is diminished and which has improved durability.
Another object of the present invention is to provide a photovoltaic element having at least a photoelectric conversion layer provided on a substrate, said photovoltaic element having a plurality of collecting electrodes spacedly arranged over a light receiving face of said photoelectric conversion layer in parallel to each other and a bus bar electrode having a desired width which is joined to said plurality of collecting electrodes to have a plurality of joining portions (A) in which said plurality of collecting electrodes are individually joined to said bus bar electrode, wherein each of said plurality of joining portions (A) has a first joining portion (i) comprising a first paste, of said plurality of collecting electrodes, each of the collecting electrodes (b) excluding the collecting electrodes (a) positioned at opposite end sides-of the arrangement of said plurality of collecting electrodes further has a second joining portion (ii) comprising a second paste having a smaller resistivity than that of said first paste at the joining portion (A) thereof with the bus bar electrode, and a proportion of the first joining portion (i) in the joining portion (A) of the collecting electrode and the bus bar electrode with respect to each of the collecting electrodes (a) is greater than a proportion of the first joining portion (i) in the joining portion (A) of the collecting electrode and the bus bar electrode with respect to each of the collecting electrodes (b).
This photovoltaic element having such electrode structure has a significant advantage in that the power loss due to the electric resistance upon collecting an electric current from the photovoltaic element is greatly diminished and has excellent durability.
In a preferred embodiment of this photovoltaic element, the collecting electrodes (a) positioned at opposite ends of the arrangement of the plurality of collecting electrodes respectively have only the first joining portion (i), each of the collecting electrodes comprises a metal wire covered by an electrically conductive paste capable of serving as the first paste, the first paste comprises a carbon paste, and the second paste comprises a paste of a metal powder.
A further object of the present invention provides a process for producing a photovoltaic element having at least a photoelectric conversion layer provided on a substrate, a plurality of collecting electrodes spacedly arranged over a light receiving face of said photoelectric conversion layer in parallel to each other and a bus bar electrode which is joined to said plurality of collecting electrodes to have a plurality of joining portions in which said plurality of collecting electrodes are individually joined to said bus bar electrode.
This production process includes the following two embodiments.
A first embodiment of the production process comprises the steps of:
(1) providing a photovoltaic body having at least a photoelectric conversion layer formed on a substrate,
(2) spacedly arranging a plurality of collecting electrodes (a) having a coat comprising a first paste over a light receiving face of said photoelectric conversion layer of said photovoltaic body in parallel to each other,
(3) of said plurality of collecting electrodes (a), removing part of the coat of each of at least the collecting electrodes (axe2x80x2) excluding the collecting electrodes (axe2x80x3) positioned on opposite end sides of the arrangement of the collecting electrodes (a),
(4) for the coat of each of the collecting electrodes (axe2x80x3), not removing part thereof or removing part thereof which is smaller than the part in said step (3),
(5) applying a second paste whose resistivity is smaller than that of the first paste to the coat-removed portion of each of the collecting electrodes whose coats are partly removed, and
(6) arranging a bus bar electrode having a desired width over the collecting electrodes (a) to cover their coat-removed portions and joining said bus bar electrode to the collecting electrodes (a) to form an electrode structure over said photoelectric conversion layer, whereby a photovoltaic element is obtained.
A second embodiment of the production process comprises the steps of:
(1) providing a photovoltaic body having a photoelectric conversion layer and a transparent electrode layer formed in this order on a substrate,
(2) forming a division groove on a light receiving face of said transparent electrode layer of said photovoltaic body in a direction perpendicular to the longitudinal direction of the substrate so that the light receiving face of the transparent electrode layer has two zones which are divided by said division glove,
(3) spacedly arranging a plurality of collecting electrodes (i) having a coat comprising a first paste over the light receiving face of the transparent electrode layer of the photovoltaic body such that said plurality of collecting electrodes are in parallel to the groove of the transparent electrode layer so that two of the collecting electrodes are positioned on opposite sides of the division groove so as to oppose to each other,
(4) of the collecting electrodes (i-a) arranged over the light receiving face of the transparent electrode layer, removing part of the coat of each of at least the collecting electrodes (i-axe2x80x2) excluding the collecting electrodes (i-axe2x80x3) positioned on the opposite sides of the division grove,
(5) for the coat of each of the collecting electrodes (i-axe2x80x3), not removing part thereof or removing part thereof which is smaller than the part in said step (4),
(6) applying a second paste whose resistivity is smaller than that of the first paste to the coat-removed portion of each of the collecting electrodes whose coats are partly removed,
(7) arranging a bus bar electrode having a desired width over the collecting electrodes (i) so as to cover their coat-removed portions and joining said bus bar electrode to the collecting electrodes (i), and
(8) cutting the substrate of the photovoltaic body along the division grove of the transparent electrode layer to obtain a plurality of photovoltaic elements.
In each of the above first and second embodiments, the step of removing part of the coat of the collecting electrode is preferred to include a step of irradiating laser beam to the coat of the collecting electrode.
The application of the second paste is preferred to be performed so that the second paste is applied in an ellipse form having a long axis in a direction of intersecting the longitudinal direction of the collecting electrode, particularly so that the second paste is applied to the coat-removed portions of the end-sided collecting electrodes in a complete round form and it is also applied to the coat-removed portions of other collecting electrodes in an ellipse form having a long axis in a direction of intersecting the longitudinal direction of the collecting electrode.